ESTRO 36 Abstract Book

S378 ESTRO 36 _______________________________________________________________________________________________

ITVs to ensure adequate coverage of remaining treatments. The adapted mid-treatment PTVs (mtPTVs) were then compared to the pretreatment PTVs (ptPTVs). Results Four patients undergoing adaptive EBRT for cervix cancer were eligible. In total, 100 CBCT images were analysed. The mean CTV volume decrease was 42% by the end of treatment, with 44% of that change (21-63%) occurring within the first week. CTV movement was patient specific, with the largest variations noted anteriorly (1.1cm to 5.5cm) and superiorly (0.67cm-4.3cm). In patients with greater than 1cm variation anteriorly and superiorly, most variation (63%-100%) occurred within the first 2 weeks of treatment.

decreased by 20%. These results support mid-treatment plan adaptation, although greater patient numbers are required for this to be validated. PO-0721 Prediction of local recurrence using pretreatment 18FDG PET/CT radiomics features in cervical cancer F. Lucia 1 , M. Desseroit 2 , O. Miranda 1 , D. Visvikis 2 , J. Malhaire 1 , P. Robin 3 , O. Pradier 1 , M. Hatt 2 , U. Schick 1 1 University Hospital- Brest- France, Radiation oncology, Brest, France 2 LaTIM- INSERM- UMR 1101- University of Brest, LaTIM- INSERM- UMR 1101- University of Brest, Brest, France 3 Nuclear Medicine department- University Hospital- Brest- France, Nuclear Medicine department- University Hospital- Brest- France, Brest, France Purpose or Objective Adequate prediction of tumor response to definitive chemoradiotherapy (CRT) in cervical cancer (CC) patients is important to offer a personalized treatment. The aim of this study was to determine if radiomics features in 18 F- fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) could help in predicting local recurrence in CC. Material and Methods Sixty-nine patients with International Federation of Gynecology and Obstetrics (FIGO) 2014 stage I2B- IVA cervical squamous cell carcinoma receiving synchronic CRT from August 2010 to May 2015 were enrolled in this study. Six (9%), nineteen (27%), thirty-five (51%) and nine (13%) women presented with FIGO stage I, II, III and IV, respectively. 18F-FDG PET/CT examination was performed on each patient before CRT which consisted of external beam radiotherapy (45 to 50.4 Gy in 1.8-2 Gy per fraction) combined with weekly platinum salts, followed by a high- dose-rate brachytherapy boost (24-26 Gy in 4 fractions). Radiomics (intensity, shape and textural) features of the primary tumor volumes delineated with the fuzzy locally adaptive bayesian (FLAB) algorithm in the PET images were extracted. For textural features three different quantization approaches were considered. The predictive value of clinical parameters and radiomics PET features regarding local recurrence-free survival (LRFS) was evaluated. Results Median follow-up was 1.8 years (0.26-5.6, range). Local relapse occurred in eleven patients (15.9%). In univariate analysis, FIGO stage (I-II vs III-IV)(p = 0.0005), T status (T1- 2 vs 3-4)(p < 0.0001), one shape feature (sphericity)(p = 0.001) and two textural features (busyness (p = 0.02), and Grey Level Non Uniformity (GLNU)(p =0.01) were significantly correlated with local recurrence. The combination of two parameters, such as busyness and sphericity (96.8 % vs 38.2%, HR, 23.51, p<0.0001) or GLNU and sphericity (94.6% vs 23.8%, HR, 20.55, p<0.0001) led to highly accurate models (sensitivity and specificity of 90.9%-77.3% and 81.8%-88.6% respectively). Conclusion In cervical cancer, radiomics features such as sphericity, busyness and GLNU from FDG PET images are significant independent predictor factors for local recurrence. Further research in refining the predictive value of these models is needed to justify dose escalation in these patients, which is already ongoing on a prospective cohort treated at our institution. PO-0722 Symptomatic pelvic insufficiency fracture in women after pelvic RT- is there a dosimetric correlate? Z. Horne 1 , M. Dohopolsky 1 , S. He 1 , B. Gill 1 , S. Beriwal 1 1 University of Pittsburgh Cancer Institute, Radiation Oncology, Pittsburgh, USA

Given that both volume and movement changes occurred predominantly within the first 2 weeks, mtPTVs were created based on a week 2 ITV. As movement was mainly superiorly and anteriorly, an anisotropic 1cm ITV margin with 0.5cm posteriorly formed the mtPTV. 93% of treatments from week 2 to 5 were covered, with 3 patients demonstrating up to a 28% reduction in volume compared to the ptPTVs. If the mtPTV was created using a 1.5cm anisotropic ITV margin (0.5cm posteriorly), treatment coverage increased to 98%, however treated volume was up to 28% larger than ptPTVs. mtPTV margins based on a week 3 ITV, showed no significant coverage difference from those based on week 2.

Reduced OAR dose was also noted, with treated bowel volume using the mtPTV decreasing by 20.7% compared

the ptPTV. Conclusion

Preliminary analyses of patients undergoing adaptive external beam treatment for cervical cancer, shows a dramatic reduction in volume and movement within the first 2 weeks of treatment. mtPTVs based on week 2 ITVs with an anisotropic 1cm margin (0.5cm posteriorly), resulted in adequate treatment coverage in more than 90% of remaining treatments. Treated bowel volume also